Gap junctional signaling in pattern regulation: Physiological network connectivity instructs growth and form

Formation of recurring transient Ca2+-based intercellular communities during Drosophila hematopoiesis

Tissue development occurs through a complex interplay between many individual cells. Yet, the fundamental question of how collective tissue behavior emerges from heterogeneous and noisy information processing and transfer at the single-cell level remains unknown. Here, we reveal that tissue scale signaling regulation can arise from local gap-junction mediated cell-cell signaling through the spatiotemporal establishment of an intermediate-scale of transient multicellular communication communities over the course of tissue development. We demonstrated this intermediate scale of emergent signaling using Ca2+ signaling in the intact, ex vivo cultured, live developing Drosophila hematopoietic organ, the lymph gland. Recurrent activation of these transient signaling communities defined self-organized signaling "hotspots" that gradually formed over the course of larva development. These hotspots receive and transmit information to facilitate repetitive interactions with nonhotspot neighbors. Overall, this work bridges the scales between single-cell and emergent group behavior providing key mechanistic insight into how cells establish tissue-scale communication networks.

Bioelectricity in dental medicine: a narrative review

BACKGROUND

Bioelectric signals, whether exogenous or endogenous, play crucial roles in the life processes of organisms. Recently, the significance of bioelectricity in the field of dentistry is steadily gaining greater attention.

OBJECTIVE

This narrative review aims to comprehensively outline the theory, physiological effects, and practical applications of bioelectricity in dental medicine and to offer insights into its potential future direction. It attempts to provide dental clinicians and researchers with an electrophysiological perspective to enhance their clinical practice or fundamental research endeavors.

METHODS

An online computer search for relevant literature was performed in PubMed, Web of Science and Cochrane Library, with the keywords "bioelectricity, endogenous electric signal, electric stimulation, dental medicine."

RESULTS

Eventually, 288 documents were included for review. The variance in ion concentration between the interior and exterior of the cell membrane, referred to as transmembrane potential, forms the fundamental basis of bioelectricity. Transmembrane potential has been established as an essential regulator of intercellular communication, mechanotransduction, migration, proliferation, and immune responses. Thus, exogenous electric stimulation can significantly alter cellular action by affecting transmembrane potential. In the field of dental medicine, electric stimulation has proven useful for assessing pulp condition, locating root apices, improving the properties of dental biomaterials, expediting orthodontic tooth movement, facilitating implant osteointegration, addressing maxillofacial malignancies, and managing neuromuscular dysfunction. Furthermore, the reprogramming of bioelectric signals holds promise as a means to guide organism development and intervene in disease processes. Besides, the development of high-throughput electrophysiological tools will be imperative for identifying ion channel targets and precisely modulating bioelectricity in the future.

CONCLUSIONS

Bioelectricity has found application in various concepts of dental medicine but large-scale, standardized, randomized controlled clinical trials are still necessary in the future. In addition, the precise, repeatable and predictable measurement and modulation methods of bioelectric signal patterns are essential research direction.

Bioelectric networks: the cognitive glue enabling evolutionary scaling from physiology to mind

Each of us made the remarkable journey from mere matter to mind: starting life as a quiescent oocyte ("just chemistry and physics"), and slowly, gradually, becoming an adult human with complex metacognitive processes, hopes, and dreams. In addition, even though we feel ourselves to be a unified, single Self, distinct from the emergent dynamics of termite mounds and other swarms, the reality is that all intelligence is collective intelligence: each of us consists of a huge number of cells working together to generate a coherent cognitive being with goals, preferences, and memories that belong to the whole and not to its parts. Basal cognition is the quest to understand how Mind scales-how large numbers of competent subunits can work together to become intelligences that expand the scale of their possible goals. Crucially, the remarkable trick of turning homeostatic, cell-level physiological competencies into large-scale behavioral intelligences is not limited to the electrical dynamics of the brain. Evolution was using bioelectric signaling long before neurons and muscles appeared, to solve the problem of creating and repairing complex bodies. In this Perspective, I review the deep symmetry between the intelligence of developmental morphogenesis and that of classical behavior. I describe the highly conserved mechanisms that enable the collective intelligence of cells to implement regulative embryogenesis, regeneration, and cancer suppression. I sketch the story of an evolutionary pivot that repurposed the algorithms and cellular machinery that enable navigation of morphospace into the behavioral navigation of the 3D world which we so readily recognize as intelligence. Understanding the bioelectric dynamics that underlie construction of complex bodies and brains provides an essential path to understanding the natural evolution, and bioengineered design, of diverse intelligences within and beyond the phylogenetic history of Earth.

Universal calcium fluctuations inHydramorphogenesis

Understanding the collective physical processes that drive robust morphological transitions in animal development necessitates the characterization of the relevant fields involved in morphogenesis. Calcium (Ca2+) is recognized as one such field. In this study, we demonstrate that the spatial fluctuations of Ca2+duringHydraregeneration exhibit universal characteristics. To investigate this phenomenon, we employ two distinct controls, an external electric field andheptanol, a gap junction-blocking drug. Both lead to the modulation of the Ca2+activity and a reversible halting of the regeneration process. The application of an electric field enhances Ca2+activity in theHydra's tissue and increases its spatial correlations, while the administration ofheptanolinhibits its activity and diminishes the spatial correlations. Remarkably, the statistical characteristics of Ca2+spatial fluctuations, including the coefficient of variation and skewness, manifest universal shape distributions across tissue samples and conditions. We introduce a field-theoretic model, describing fluctuations in a tilted double-well potential, which successfully captures these universal properties. Moreover, our analysis reveals that the Ca2+activity is spatially localized, and theHydra's tissue operates near the onset of bistability, where the local Ca2+activity fluctuates between low and high excited states in distinct regions. These findings highlight the prominent role of the Ca2+field inHydramorphogenesis and provide insights into the underlying mechanisms governing robust morphological transitions.

Future medicine: from molecular pathways to the collective intelligence of the body

The remarkable anatomical homeostasis exhibited by complex living organisms suggests that they are inherently reprogrammable information-processing systems that offer numerous interfaces to their physiological and anatomical problem-solving capacities. We briefly review data suggesting that the multiscale competency of living forms affords a new path for biomedicine that exploits the innate collective intelligence of tissues and organs. The concept of tissue-level allostatic goal-directedness is already bearing fruit in clinical practice. We sketch a roadmap towards 'somatic psychiatry' by using advances in bioelectricity and behavioral neuroscience to design methods that induce self-repair of structure and function. Relaxing the assumption that cellular control mechanisms are static, exploiting powerful concepts from cybernetics, behavioral science, and developmental biology may spark definitive solutions to current biomedical challenges.

The Yin and Yang of Breast Cancer: Ion Channels as Determinants of Left-Right Functional Differences

Breast cancer is a complex and heterogeneous disease that displays diverse molecular subtypes and clinical outcomes. Although it is known that the location of tumors can affect their biological behavior, the underlying mechanisms are not fully understood. In our previous study, we found a differential methylation profile and membrane potential between left (L)- and right (R)-sided breast tumors. In this current study, we aimed to identify the ion channels responsible for this phenomenon and determine any associated phenotypic features. To achieve this, experiments were conducted in mammary tumors in mice, human patient samples, and with data from public datasets. The results revealed that L-sided tumors have a more depolarized state than R-sided. We identified a 6-ion channel-gene signature (CACNA1C, CACNA2D2, CACNB2, KCNJ11, SCN3A, and SCN3B) associated with the side: L-tumors exhibit lower expression levels than R-tumors. Additionally, in silico analyses show that the signature correlates inversely with DNA methylation writers and with key biological processes involved in cancer progression, such as proliferation and stemness. The signature also correlates inversely with patient survival rates. In an in vivo mouse model, we confirmed that KI67 and CD44 markers were increased in L-sided tumors and a similar tendency for KI67 was found in patient L-tumors. Overall, this study provides new insights into the potential impact of anatomical location on breast cancer biology and highlights the need for further investigation into possible differential treatment options.

Electrochemical Devices in Cutaneous Wound Healing

In healthy skin, vectorial ion transport gives rise to a transepithelial potential which directly impacts many physiological aspects of skin function. A wound is a physical defect that breaches the epithelial barrier and changes the electrochemical environment of skin. Electroceutical dressings are devices that manipulate the electrochemical environment, host as well as microbial, of a wound. In this review, electroceuticals are organized into three mechanistic classes: ionic, wireless, and battery powered. All three classes of electroceutical dressing show encouraging effects on infection management and wound healing with evidence of favorable impact on keratinocyte migration and disruption of wound biofilm infection. This foundation sets the stage for further mechanistic as well as interventional studies. Successful conduct of such studies will determine the best dosage, timing, and class of stimulus necessary to maximize therapeutic efficacy.

The scaling of goals from cellular to anatomical homeostasis: an evolutionary simulation, experiment and analysis

Complex living agents consist of cells, which are themselves competent sub-agents navigating physiological and metabolic spaces. Behaviour science, evolutionary developmental biology and the field of machine intelligence all seek to understand the scaling of biological cognition: what enables individual cells to integrate their activities to result in the emergence of a novel, higher-level intelligence with large-scale goals and competencies that belong to it and not to its parts? Here, we report the results of simulations based on the TAME framework, which proposes that evolution pivoted the collective intelligence of cells during morphogenesis of the body into traditional behavioural intelligence by scaling up homeostatic competencies of cells in metabolic space. In this article, we created a minimal in silico system (two-dimensional neural cellular automata) and tested the hypothesis that evolutionary dynamics are sufficient for low-level setpoints of metabolic homeostasis in individual cells to scale up to tissue-level emergent behaviour. Our system showed the evolution of the much more complex setpoints of cell collectives (tissues) that solve a problem in morphospace: the organization of a body-wide positional information axis (the classic French flag problem in developmental biology). We found that these emergent morphogenetic agents exhibit a number of predicted features, including the use of stress propagation dynamics to achieve the target morphology as well as the ability to recover from perturbation (robustness) and long-term stability (even though neither of these was directly selected for). Moreover, we observed an unexpected behaviour of sudden remodelling long after the system stabilizes. We tested this prediction in a biological system-regenerating planaria-and observed a very similar phenomenon. We propose that this system is a first step towards a quantitative understanding of how evolution scales minimal goal-directed behaviour (homeostatic loops) into higher-level problem-solving agents in morphogenetic and other spaces.

Cellular signaling pathways as plastic, proto-cognitive systems: Implications for biomedicine

Many aspects of health and disease are modeled using the abstraction of a "pathway"-a set of protein or other subcellular activities with specified functional linkages between them. This metaphor is a paradigmatic case of a deterministic, mechanistic framework that focuses biomedical intervention strategies on altering the members of this network or the up-/down-regulation links between them-rewiring the molecular hardware. However, protein pathways and transcriptional networks exhibit interesting and unexpected capabilities such as trainability (memory) and information processing in a context-sensitive manner. Specifically, they may be amenable to manipulation via their history of stimuli (equivalent to experiences in behavioral science). If true, this would enable a new class of biomedical interventions that target aspects of the dynamic physiological "software" implemented by pathways and gene-regulatory networks. Here, we briefly review clinical and laboratory data that show how high-level cognitive inputs and mechanistic pathway modulation interact to determine outcomes in vivo. Further, we propose an expanded view of pathways from the perspective of basal cognition and argue that a broader understanding of pathways and how they process contextual information across scales will catalyze progress in many areas of physiology and neurobiology. We argue that this fuller understanding of the functionality and tractability of pathways must go beyond a focus on the mechanistic details of protein and drug structure to encompass their physiological history as well as their embedding within higher levels of organization in the organism, with numerous implications for data science addressing health and disease. Exploiting tools and concepts from behavioral and cognitive sciences to explore a proto-cognitive metaphor for the pathways underlying health and disease is more than a philosophical stance on biochemical processes; at stake is a new roadmap for overcoming the limitations of today's pharmacological strategies and for inferring future therapeutic interventions for a wide range of disease states.

A computational model of cell membrane bioelectric polarization and depolarization, connected with cell proliferation, in different tissue geometries

A reliable theory of biological tissues growth and organization, a fundamental tool for a comprehensive interpretation of experimental observations and a guide to progress in life sciences, is definitively missing. This would support the advancement of knowledge for both normal and pathological expansion and regulation of tissues and organisms. In this work is presented a computational model of cell culture that describes its growth and organization using cell proliferation as its default state, constrained by contact inhibition, closely connected to the cell bioelectric state. The model results describe in a correct way the reported experimental results, involving contact inhibition due to the presence of other cells, and gap junctions for signaling, molecules exchange and extracellular environment sensing. Starting from depolarized cells (in this model considered tantamount to proliferative), the cell culture grows until it fills the available domain and, due to the contact inhibition constraint, it turns into quiescence (a consequence of cell polarization), except on the periphery. Using drugs or via protein expression manipulation, it is possible to change the final tissue state, to fully polarized or depolarized. Other experimental tests are proposed and the expected results simulated. This model can be extended to pathological events, such as carcinogenesis, with cells homeostasis perturbed by a cell depolarizing (carcinogenic) event and express its default proliferative state without adequate control. This simplified model of tissue organization, regulated by the cell's bioelectric state and their interaction with vicinity, is an alternative to the description of the experimental results by mechanical stress, and can be further tested and extended in dedicated experiments.

Leaf vein patterning is regulated by the aperture of plasmodesmata intercellular channels

To form tissue networks, animal cells migrate and interact through proteins protruding from their plasma membranes. Plant cells can do neither, yet plants form vein networks. How plants do so is unclear, but veins are thought to form by the coordinated action of the polar transport and signal transduction of the plant hormone auxin. However, plants inhibited in both pathways still form veins. Patterning of vascular cells into veins is instead prevented in mutants lacking the function of the GNOM (GN) regulator of auxin transport and signaling, suggesting the existence of at least one more GN-dependent vein-patterning pathway. Here we show that in Arabidopsis such a pathway depends on the movement of auxin or an auxin-dependent signal through plasmodesmata (PDs) intercellular channels. PD permeability is high where veins are forming, lowers between veins and nonvascular tissues, but remains high between vein cells. Impaired ability to regulate PD aperture leads to defects in auxin transport and signaling, ultimately leading to vein patterning defects that are enhanced by inhibition of auxin transport or signaling. GN controls PD aperture regulation, and simultaneous inhibition of auxin signaling, auxin transport, and regulated PD aperture phenocopies null gn mutants. Therefore, veins are patterned by the coordinated action of three GN-dependent pathways: auxin signaling, polar auxin transport, and movement of auxin or an auxin-dependent signal through PDs. Such a mechanism of tissue network formation is unprecedented in multicellular organisms.

How do plants form vein networks, in the absence of cellular migration or direct cell-cell interaction? This study shows that a GNOM-dependent combination of polar auxin transport, auxin signal transduction, and movement of an auxin signal through plasmodesmata patterns leaf vascular cells into veins.

Bioelectric Dysregulation in Cancer Initiation, Promotion, and Progression

Cancer is primarily a disease of dysregulation – both at the genetic level and at the tissue organization level. One way that tissue organization is dysregulated is by changes in the bioelectric regulation of cell signaling pathways. At the basis of bioelectricity lies the cellular membrane potential or V_mem, an intrinsic property associated with any cell. The bioelectric state of cancer cells is different from that of healthy cells, causing a disruption in the cellular signaling pathways. This disruption or dysregulation affects all three processes of carcinogenesis – initiation, promotion, and progression. Another mechanism that facilitates the homeostasis of cell signaling pathways is the production of extracellular vesicles (EVs) by cells. EVs also play a role in carcinogenesis by mediating cellular communication within the tumor microenvironment (TME). Furthermore, the production and release of EVs is altered in cancer. To this end, the change in cell electrical state and in EV production are responsible for the bioelectric dysregulation which occurs during cancer. This paper reviews the bioelectric dysregulation associated with carcinogenesis, including the TME and metastasis. We also look at the major ion channels associated with cancer and current technologies and tools used to detect and manipulate bioelectric properties of cells.

A Computational Approach to Explaining Bioelectrically Induced Persistent, Stochastic Changes of Axial Polarity in Planarian Regeneration

Morphogenesis results when cells cooperate to construct a specific anatomical structure. The behavior of such cell swarms exhibits not only robustness but also plasticity with respect to what specific anatomies will be built. Important aspects of evolutionary biology, regenerative medicine, and cancer are impacted by the algorithms by which instructive information guides invariant or stochastic outcomes of such collective activity. Planarian flatworms are an important model system in this field, as flatworms reliably regenerate a primary body axis after diverse kinds of injury. Importantly, the number of heads to which they regenerate is not determined genetically: lines of worms can be produced, which, with no further manipulation, regenerate as two-headed (2H) worms, or as "Cryptic" worms. When cut into pieces, Cryptic worms produce one-headed (1H) and 2H regenerates stochastically. Neural and bioelectric mechanisms have been proposed to explain aspects of the regenerative dataset. However, these models have not been unified and do not explain all of the Cryptic worm data. In this study, we propose a model in which two separate systems (a bioelectric circuit and a neural polarity mechanism) compete to determine the anatomical structure of a regenerate. We show how our model accounts for existing data and provides a consistent synthesis of mechanisms that explain both the robustness of planarian regeneration and its remarkable re-writability toward novel stable and stochastic anatomical states.

Minimal Developmental Computation: A Causal Network Approach to Understand Morphogenetic Pattern Formation

What information-processing strategies and general principles are sufficient to enable self-organized morphogenesis in embryogenesis and regeneration? We designed and analyzed a minimal model of self-scaling axial patterning consisting of a cellular network that develops activity patterns within implicitly set bounds. The properties of the cells are determined by internal 'genetic' networks with an architecture shared across all cells. We used machine-learning to identify models that enable this virtual mini-embryo to pattern a typical axial gradient while simultaneously sensing the set boundaries within which to develop it from homogeneous conditions-a setting that captures the essence of early embryogenesis. Interestingly, the model revealed several features (such as planar polarity and regenerative re-scaling capacity) for which it was not directly selected, showing how these common biological design principles can emerge as a consequence of simple patterning modes. A novel "causal network" analysis of the best model furthermore revealed that the originally symmetric model dynamically integrates into intercellular causal networks characterized by broken-symmetry, long-range influence and modularity, offering an interpretable macroscale-circuit-based explanation for phenotypic patterning. This work shows how computation could occur in biological development and how machine learning approaches can generate hypotheses and deepen our understanding of how featureless tissues might develop sophisticated patterns-an essential step towards predictive control of morphogenesis in regenerative medicine or synthetic bioengineering contexts. The tools developed here also have the potential to benefit machine learning via new forms of backpropagation and by leveraging the novel distributed self-representation mechanisms to improve robustness and generalization.

Cell Systems Bioelectricity: How Different Intercellular Gap Junctions Could Regionalize a Multicellular Aggregate

Electric potential distributions can act as instructive pre-patterns for development, regeneration, and tumorigenesis in cell systems. The biophysical states influence transcription, proliferation, cell shape, migration, and differentiation through biochemical and biomechanical downstream transduction processes. A major knowledge gap is the origin of spatial patterns in vivo, and their relationship to the ion channels and the electrical synapses known as gap junctions. Understanding this is critical for basic evolutionary developmental biology as well as for regenerative medicine. We computationally show that cells may express connexin proteins with different voltage-gated gap junction conductances as a way to maintain multicellular regions at distinct membrane potentials. We show that increasing the multicellular connectivity via enhanced junction function does not always contribute to the bioelectrical normalization of abnormally depolarized multicellular patches. From a purely electrical junction view, this result suggests that the reduction rather than the increase of specific connexin levels can also be a suitable bioelectrical approach in some cases and time stages. We offer a minimum model that incorporates effective conductances ultimately related to specific ion channel and junction proteins that are amenable to external regulation. We suggest that the bioelectrical patterns and their encoded instructive information can be externally modulated by acting on the mean fields of cell systems, a complementary approach to that of acting on the molecular characteristics of individual cells. We believe that despite the limitations of a biophysically focused model, our approach can offer useful qualitative insights into the collective dynamics of cell system bioelectricity.

Beyond Neurons: Long Distance Communication in Development and Cancer

Cellular communication is important in all aspects of tissue and organism functioning, from the level of single cells, two discreet populations, and distant tissues of the body. Long distance communication networks integrate individual cells into tissues to maintain a complex organism during development, but when communication between cells goes awry, disease states such as cancer emerge. Herein we discuss the growing body of evidence suggesting that communication methods known to be employed by neurons, also exist in other cell types. We identify three major areas of long-distance communication: bioelectric signaling, tunneling nanotubes (TNTs), and macrophage modulation of networks, and draw comparisons about how these systems operate in the context of development and cancer. Bioelectric signaling occurs between cells through exchange of ions and tissue-level electric fields, leading to changes in biochemical gradients and molecular signaling pathways to control normal development and tumor growth and invasion in cancer. TNTs transport key morphogens and other cargo long distances, mediating electrical coupling, tissue patterning, and malignancy of cancer cells. Lastly macrophages maintain long distance signaling networks through trafficking of vesicles during development, providing communication relays and priming favorable microenvironments for cancer metastasis. By drawing comparisons between non-neural long distance signaling in the context of development and cancer we aim to encourage crosstalk between the two fields to cultivate new hypotheses and potential therapeutic strategies.

Morphology changes induced by intercellular gap junction blocking: A reaction-diffusion mechanism

Complex anatomical form is regulated in part by endogenous physiological communication between cells; however, the dynamics by which gap junctional (GJ) states across tissues regulate morphology are still poorly understood. We employed a biophysical modeling approach combining different signaling molecules (morphogens) to qualitatively describe the anteroposterior and lateral morphology changes in model multicellular systems due to intercellular GJ blockade. The model is based on two assumptions for blocking-induced patterning: (i) the local concentrations of two small antagonistic morphogens diffusing through the GJs along the axial direction, together with that of an independent, uncoupled morphogen concentration along an orthogonal direction, constitute the instructive patterns that modulate the morphological outcomes, and (ii) the addition of an external agent partially blocks the intercellular GJs between neighboring cells and modifies thus the establishment of these patterns. As an illustrative example, we study how the different connectivity and morphogen patterns obtained in presence of a GJ blocker can give rise to novel head morphologies in regenerating planaria. We note that the ability of GJs to regulate the permeability of morphogens post-translationally suggests a mechanism by which different anatomies can be produced from the same genome without the modification of gene-regulatory networks. Conceptually, our model biosystem constitutes a reaction-diffusion information processing mechanism that allows reprogramming of biological morphologies through the external manipulation of the intercellular GJs and the resulting changes in instructive biochemical signals.

A Meta-Analysis of Bioelectric Data in Cancer, Embryogenesis, and Regeneration

Developmental bioelectricity is the study of the endogenous role of bioelectrical signaling in all cell types. Resting potentials and other aspects of ionic cell physiology are known to be important regulatory parameters in embryogenesis, regeneration, and cancer. However, relevant quantitative measurement and genetic phenotyping data are distributed throughout wide-ranging literature, hampering experimental design and hypothesis generation. Here, we analyze published studies on bioelectrics and transcriptomic and genomic/phenotypic databases to provide a novel synthesis of what is known in three important aspects of bioelectrics research. First, we provide a comprehensive list of channelopathies-ion channel and pump gene mutations-in a range of important model systems with developmental patterning phenotypes, illustrating the breadth of channel types, tissues, and phyla (including man) in which bioelectric signaling is a critical endogenous aspect of embryogenesis. Second, we perform a novel bioinformatic analysis of transcriptomic data during regeneration in diverse taxa that reveals an electrogenic protein to be the one common factor specifically expressed in regeneration blastemas across Kingdoms. Finally, we analyze data on distinct V_mem signatures in normal and cancer cells, revealing a specific bioelectrical signature corresponding to some types of malignancies. These analyses shed light on fundamental questions in developmental bioelectricity and suggest new avenues for research in this exciting field.

Optical Imaging of Electrical and Mechanical Couplings between Cells

Intercellular communication plays a pivotal role in multicellular organisms. Studying the electrical and mechanical coupling among multiple cells has been a difficult task due to the lack of suitable techniques. In this study, we developed a label-free imaging method for monitoring the electrical-induced communications between connected cells. The method was based on monitoring subtle mechanical motions of the cells under electrical modulation of the membrane potential. We observed that connected cells responded to electrical modulation of neighboring cells with mechanical deformation of the membrane. We further investigated the mechanism of the coupling and confirmed that this mechanical response was induced by electrical signal communicated through the gap junction. Blocking the gap junction can temporally cease the mechanical signal, and this inhibition can be rescued after removing the inhibitor. This study sheds light on the mechanism of electrical coupling between neurons and provides a new method for studying intercellular communications.

Rewiring Endogenous Bioelectric Circuits in the Xenopus laevis Embryo Model

Embryogenesis, as well as regeneration, is increasingly recognized to be orchestrated by an interplay of transcriptional and bioelectric networks. Spatiotemporal patterns of resting potentials direct the size, shape, and locations of numerous organ primordia during patterning. These bioelectrical properties are established by the function of ion channels and pumps that set voltage potentials of individual cells, and gap junctions (electrical synapses) that enable physiological states to propagate across tissue networks. Functional experiments to probe the roles of bioelectrical states can be carried out by targeting endogenous ion channels during development. Here, we describe protocols, optimized for the highly tractable Xenopus laevis embryo, for molecular genetic targeting of ion channels and connexins based on CRISPR, and monitoring of resting potential states using voltage-sensing fluorescent dye. Similar strategies can be adapted to other model species.

Community effects allow bioelectrical reprogramming of cell membrane potentials in multicellular aggregates: Model simulations

Bioelectrical patterns are established by spatiotemporal correlations of cell membrane potentials at the multicellular level, being crucial to development, regeneration, and tumorigenesis. We have conducted multicellular simulations on bioelectrical community effects and intercellular coupling in multicellular aggregates. The simulations aim at establishing under which conditions a local heterogeneity consisting of a small patch of cells can be stabilized against a large aggregate of surrounding identical cells which are in a different bioelectrical state. In this way, instructive bioelectrical information can be persistently encoded in spatiotemporal patterns of separated domains with different cell polarization states. The multicellular community effects obtained are regulated both at the single-cell and intercellular levels, and emerge from a delicate balance between the degrees of intercellular coupling in: (i) the small patch, (ii) the surrounding bulk, and (iii) the interface that separates these two regions. The model is experimentally motivated and consists of two generic voltage-gated ion channels that attempt to establish the depolarized and polarized cell states together with coupling conductances whose individual and intercellular different states permit a dynamic multicellular connectivity. The simulations suggest that community effects may allow the reprogramming of single-cell bioelectrical states, in agreement with recent experimental data. A better understanding of the resulting electrical regionalization can assist the electroceutical correction of abnormally depolarized regions initiated in the bulk of normal tissues as well as suggest new biophysical mechanisms for the establishment of target patterns in multicellular engineering.

Connexin 43 gap junctional intercellular communication inhibits evx1 expression and joint formation in regenerating fins

The gap junction protein Connexin 43 (Cx43) contributes to cell fate decisions that determine the location of fin ray joints during regeneration. Here, we provide insights into how Cx43, expressed medially, influences changes in gene expression in lateral skeletal precursor cells. Using the Gap27 peptide inhibitor specific to Cx43, we show that Cx43-gap junctional intercellular communication (GJIC) influences Cx43-dependent skeletal phenotypes, including segment length. We also demonstrate that Cx43-GJIC influences the expression of the Smp/β-catenin pathway in the lateral skeletal precursor cells, and does not influence the Sema3d pathway. Moreover, we show that the cx43^lh10 allele, which has increased Cx43 protein levels, exhibits increased regenerate length and segment length. These phenotypes are rescued by Gap27, suggesting that increased Cx43 is responsible for the observed Cx43 phenotypes. Finally, our findings suggest that inhibition of Cx43 hemichannel activity does not influence Cx43-dependent skeletal phenotypes. These data provide evidence that Cx43-GJIC is responsible for regulating cell fate decisions associated with appropriate joint formation in the regenerating fin.

Epithelial tissue geometry directs emergence of bioelectric field and pattern of proliferation

Patterns of proliferation are templated by both gradients of mechanical stress as well as by gradients in membrane voltage (Vm), which is defined as the electric potential difference between the cytoplasm and the extracellular medium. Either gradient could regulate the emergence of the other, or they could arise independently and synergistically affect proliferation within a tissue. Here, we examined the relationship between endogenous patterns of mechanical stress and the generation of bioelectric gradients in mammary epithelial tissues. We observed that the mechanical stress gradients in the tissues presaged gradients in both proliferation and depolarization, consistent with previous reports correlating depolarization with proliferation. Furthermore, disrupting the Vm gradient blocked the emergence of patterned proliferation. We found that the bioelectric gradient formed downstream of mechanical stresses within the tissues and depended on connexin-43 (Cx43) hemichannels, which opened preferentially in cells located in regions of high mechanical stress. Activation of Cx43 hemichannels was necessary for nuclear localization of Yap/Taz and induction of proliferation. Together, these results suggest that mechanotransduction triggers the formation of bioelectric gradients across a tissue, which are further translated into transcriptional changes that template patterns of growth.

HCN2 Channel-Induced Rescue of Brain Teratogenesis via Local and Long-Range Bioelectric Repair

Embryonic exposure to the teratogen nicotine results in brain defects, by disrupting endogenous spatial pre patterns necessary for normal brain size and patterning. Extending prior work in Xenopus laevis that showed that misexpression of ion channels can rescue morphogenesis, we demonstrate and characterize a novel aspect of developmental bioelectricity: channel-dependent repair signals propagate long-range across the embryo. We show that distal HCN2 channel misexpression and distal transplants of HCN2-expressing tissue, non-cell-autonomously reverse profound defects, rescuing brain anatomy, gene expression, and learning. Moreover, such rescue can be induced by small-molecule HCN2 channel activators, even with delayed treatment initiation. We present a simple, versatile computational model of bioelectrical signaling upstream of key patterning genes such as OTX2 and XBF1, which predicts long-range repair induced by ion channel activity, and experimentally validate the predictions of this model. Our results and quantitative model identify a powerful morphogenetic control mechanism that could be targeted by future regenerative medicine exploiting ion channel modulating drugs approved for human use.

Bioelectrical Coupling of Single-Cell States in Multicellular Systems

The spatiotemporal distributions of signaling ions and molecules that modulate biochemical pathways in nonexcitable cells are influenced by multicellular electric potentials. These potentials act as distributed controllers encoding instructive spatial patterns in development and regeneration. We review experimental facts and discuss recent bioelectrical models that provide new physical insights and complement biochemical approaches. Single-cell states are modulated at the multicellular level because of the coupling between neighboring cells, thus allowing memories and multicellular patterns. The model is based on (i) two generic voltage-gated ion channels that promote the polarized and depolarized cell states, (ii) a feedback mechanism for the transcriptional and bioelectrical regulations, and (iii) voltage-gated intercellular conductances that allow a dynamic intercellular connectivity. The simulations provide steady-state and oscillatory multicellular states that help explain aspects of development and guide experimental procedures attempting to establish instructive bioelectrical patterns based on electric potentials and currents to regulate cell behavior and morphogenesis.

Bioelectrical and cytoskeletal patterns correlate with altered axial polarity in the follicular epithelium of the Drosophila mutant gurken

Background

Bioelectrical signals are known to be involved in the generation of cell and tissue polarity as well as in cytoskeletal dynamics. The epithelium of Drosophila ovarian follicles is a suitable model system for studying connections between electrochemical gradients, patterns of cytoskeletal elements and axial polarity. By interactions between soma and germline cells, the transforming growth factor-α homolog Gurken (Grk) establishes both the anteroposterior and the dorsoventral axis during oogenesis.

Results

In the follicular epithelium of the wild-type (wt) and the polarity mutant grk, we analysed stage-specific gradients of membrane potentials (V_mem) and intracellular pH (pH_i) using the potentiometric dye DiBAC₄(3) and the fluorescent pH-indicator 5-CFDA,AM, respectively. In addition, we compared the cytoskeletal organisation in the follicular epithelium of wt and grk using fluorescent phalloidin and an antibody against acetylated α-tubulin. Corresponding to impaired polarity in grk, the slope of the anteroposterior V_mem-gradient in stage S9 is significantly reduced compared to wt. Even more striking differences in V_mem- and pH_i-patterns become obvious during stage S10B, when the respective dorsoventral gradients are established in wt but not in grk. Concurrent with bioelectrical differences, wt and grk exhibit differences concerning cytoskeletal patterns in the follicular epithelium. During all vitellogenic stages, basal microfilaments in grk are characterised by transversal alignment, while wt-typical condensations in centripetal follicle cells (S9) and in dorsal centripetal follicle cells (S10B) are absent. Moreover, in grk, longitudinal alignment of microtubules occurs throughout vitellogenesis in all follicle cells, whereas in wt, microtubules in mainbody and posterior follicle cells exhibit a more cell-autonomous organisation. Therefore, in contrast to wt, the follicular epithelium in grk is characterised by missing or shallower electrochemical gradients and by more coordinated transcellular cytoskeletal patterns.

Conclusions

Our results show that bioelectrical polarity and cytoskeletal polarity are closely linked to axial polarity in both wt and grk. When primary polarity signals are altered, both bioelectrical and cytoskeletal patterns in the follicular epithelium change. We propose that not only cell-specific levels of V_mem and pH_i, or the polarities of transcellular electrochemical gradients, but also the slopes of these gradients are crucial for cytoskeletal modifications and, thus, for proper development of epithelial polarity.

Interplay between morphogen-directed positional information systems and physiological signaling

The development of an organism from an undifferentiated single cell into a spatially complex structure requires spatial patterning of cell fates across tissues. Positional information, proposed by Lewis Wolpert in 1969, has led to the characterization of many components involved in regulating morphogen signaling activity. However, how morphogen gradients are established, maintained, and interpreted by cells still is not fully understood. Quantitative and systems-based approaches are increasingly needed to define general biological design rules that govern positional information systems in developing organisms. This short review highlights a selective set of studies that have investigated the roles of physiological signaling in modulating and mediating morphogen-based pattern formation. Similarities between neural transmission and morphogen-based pattern formation mechanisms suggest underlying shared principles of active cell-based communication. Within larger tissues, neural networks provide directed information, via physiological signaling, that supplements positional information through diffusion. Further, mounting evidence demonstrates that physiological signaling plays a role in ensuring robustness of morphogen-based signaling. We conclude by highlighting several outstanding questions regarding the role of physiological signaling in morphogen-based pattern formation. Elucidating how physiological signaling impacts positional information is critical for understanding the close coupling of developmental and cellular processes in the context of development, disease, and regeneration.

Endogenous Bioelectrics in Development, Cancer, and Regeneration: Drugs and Bioelectronic Devices as Electroceuticals for Regenerative Medicine

A major frontier in the post-genomic era is the investigation of the control of coordinated growth and three-dimensional form. Dynamic remodeling of complex organs in regulative embryogenesis, regeneration, and cancer reveals that cells and tissues make decisions that implement complex anatomical outcomes. It is now essential to understand not only the genetics that specifies cellular hardware but also the physiological software that implements tissue-level plasticity and robust morphogenesis. Here, we review recent discoveries about the endogenous mechanisms of bioelectrical communication among non-neural cells that enables them to cooperate in vivo. We discuss important advances in bioelectronics, as well as computational and pharmacological tools that are enabling the taming of biophysical controls toward applications in regenerative medicine and synthetic bioengineering.

The Computational Boundary of a "Self": Developmental Bioelectricity Drives Multicellularity and Scale-Free Cognition

All epistemic agents physically consist of parts that must somehow comprise an integrated cognitive self. Biological individuals consist of subunits (organs, cells, and molecular networks) that are themselves complex and competent in their own native contexts. How do coherent biological Individuals result from the activity of smaller sub-agents? To understand the evolution and function of metazoan creatures' bodies and minds, it is essential to conceptually explore the origin of multicellularity and the scaling of the basal cognition of individual cells into a coherent larger organism. In this article, I synthesize ideas in cognitive science, evolutionary biology, and developmental physiology toward a hypothesis about the origin of Individuality: "Scale-Free Cognition." I propose a fundamental definition of an Individual based on the ability to pursue goals at an appropriate level of scale and organization and suggest a formalism for defining and comparing the cognitive capacities of highly diverse types of agents. Any Self is demarcated by a computational surface - the spatio-temporal boundary of events that it can measure, model, and try to affect. This surface sets a functional boundary - a cognitive "light cone" which defines the scale and limits of its cognition. I hypothesize that higher level goal-directed activity and agency, resulting in larger cognitive boundaries, evolve from the primal homeostatic drive of living things to reduce stress - the difference between current conditions and life-optimal conditions. The mechanisms of developmental bioelectricity - the ability of all cells to form electrical networks that process information - suggest a plausible set of gradual evolutionary steps that naturally lead from physiological homeostasis in single cells to memory, prediction, and ultimately complex cognitive agents, via scale-up of the basic drive of infotaxis. Recent data on the molecular mechanisms of pre-neural bioelectricity suggest a model of how increasingly sophisticated cognitive functions emerge smoothly from cell-cell communication used to guide embryogenesis and regeneration. This set of hypotheses provides a novel perspective on numerous phenomena, such as cancer, and makes several unique, testable predictions for interdisciplinary research that have implications not only for evolutionary developmental biology but also for biomedicine and perhaps artificial intelligence and exobiology.

Modeling somatic computation with non-neural bioelectric networks

The field of basal cognition seeks to understand how adaptive, context-specific behavior occurs in non-neural biological systems. Embryogenesis and regeneration require plasticity in many tissue types to achieve structural and functional goals in diverse circumstances. Thus, advances in both evolutionary cell biology and regenerative medicine require an understanding of how non-neural tissues could process information. Neurons evolved from ancient cell types that used bioelectric signaling to perform computation. However, it has not been shown whether or how non-neural bioelectric cell networks can support computation. We generalize connectionist methods to non-neural tissue architectures, showing that a minimal non-neural Bio-Electric Network (BEN) model that utilizes the general principles of bioelectricity (electrodiffusion and gating) can compute. We characterize BEN behaviors ranging from elementary logic gates to pattern detectors, using both fixed and transient inputs to recapitulate various biological scenarios. We characterize the mechanisms of such networks using dynamical-systems and information-theory tools, demonstrating that logic can manifest in bidirectional, continuous, and relatively slow bioelectrical systems, complementing conventional neural-centric architectures. Our results reveal a variety of non-neural decision-making processes as manifestations of general cellular biophysical mechanisms and suggest novel bioengineering approaches to construct functional tissues for regenerative medicine and synthetic biology as well as new machine learning architectures.

Electrochemical gradients are involved in regulating cytoskeletal patterns during epithelial morphogenesis in the Drosophila ovary

BACKGROUND

During Drosophila oogenesis, the follicular epithelium differentiates into several morphologically distinct follicle-cell populations. Characteristic bioelectrical properties make this tissue a suitable model system for studying connections between electrochemical signals and the organisation of the cytoskeleton. Recently, we have described stage-specific transcellular antero-posterior and dorso-ventral gradients of intracellular pH (pH_i) and membrane potential (V_mem) depending on the asymmetrical distribution and/or activity of various ion-transport mechanisms. In the present study, we analysed the patterns of basal microfilaments (bMF) and microtubules (MT) in relation to electrochemical signals.

RESULTS

The bMF- and MT-patterns in developmental stages 8 to 12 were visualised using labelled phalloidin and an antibody against acetylated α-tubulin as well as follicle-cell specific expression of GFP-actin and GFP-α-tubulin. Obviously, stage-specific changes of the pH_i- and V_mem-gradients correlate with modifications of the bMF- and MT-organisation. In order to test whether cytoskeletal modifications depend directly on bioelectrical changes, we used inhibitors of ion-transport mechanisms that have previously been shown to modify pH_i and V_mem as well as the respective gradients. We inhibited, in stage 10b, Na⁺/H⁺-exchangers and Na⁺-channels with amiloride, V-ATPases with bafilomycin, ATP-sensitive K⁺-channels with glibenclamide, voltage-dependent L-type Ca²⁺-channels with verapamil, Cl^--channels with 9-anthroic acid and Na⁺/K⁺/2Cl^--cotransporters with furosemide, respectively. The correlations between pH_i, V_mem, bMF and MT observed in different follicle-cell types are in line with the correlations resulting from the inhibition experiments. While relative alkalisation and/or hyperpolarisation stabilised the parallel transversal alignment of bMF, acidification led to increasing disorder and to condensations of bMF. On the other hand, relative acidification as well as hyperpolarisation stabilised the longitudinal orientation of MT, whereas alkalisation led to loss of this arrangement and to partial disintegration of MT.

CONCLUSIONS

We conclude that the pH_i- and V_mem-changes induced by inhibitors of ion-transport mechanisms simulate bioelectrical changes occurring naturally and leading to the cytoskeletal changes observed during differentiation of the follicle-cell epithelium. Therefore, gradual modifications of electrochemical signals can serve as physiological means to regulate cell and tissue architecture by modifying cytoskeletal patterns.

Gaps and barriers: Gap junctions as a channel of communication between the soma and the germline

Gap junctions, expressed in most tissues of the body, allow for the cytoplasmic coupling of adjacent cells and promote tissue cooperation. Gap junctions connect also the soma and the germline in many animals, and transmit somatic signals that are crucial for germline maturation and integrity. In this review, we examine the involvement of gap junctions in the relay of information between the soma and the germline, and ask whether such communication could have consequences for the progeny. While the influence of parental experiences on descendants is of great interest, the possibility that gap junctions participate in the transmission of information across generations is largely unexplored.

The N-space Episenome unifies cellular information space-time within cognition-based evolution

Self-referential cellular homeostasis is maintained by the measured assessment of both internal status and external conditions based within an integrated cellular information field. This cellular field attachment to biologic information space-time coordinates environmental inputs by connecting the cellular senome, as the sum of the sensory experiences of the cell, with its genome and epigenome. In multicellular organisms, individual cellular information fields aggregate into a collective information architectural matrix, termed a N-space Episenome, that enables mutualized organism-wide information management. It is hypothesized that biological organization represents a dual heritable system constituted by both its biological materiality and a conjoining N-space Episenome. It is further proposed that morphogenesis derives from reciprocations between these inter-related facets to yield coordinated multicellular growth and development. The N-space Episenome is conceived as a whole cell informational projection that is heritable, transferable via cell division and essential for the synchronous integration of the diverse self-referential cells that constitute holobionts.

From non-excitable single-cell to multicellular bioelectrical states supported by ion channels and gap junction proteins: Electrical potentials as distributed controllers

Endogenous bioelectric patterns within tissues are an important driver of morphogenesis and a tractable component of a number of disease states. Developing system-level understanding of the dynamics by which non-neural bioelectric circuits regulate complex downstream cascades is a key step towards both, an evolutionary understanding of ion channel genes, and novel strategies in regenerative medicine. An important capability gap is deriving rational modulation strategies targeting individual cells' bioelectric states to achieve global (tissue- or organ-level) outcomes. Here, we develop an ion channel-based model that describes multicellular states on the basis of spatio-temporal patterns of electrical potentials in aggregates of non-excitable cells. The model is of biological interest because modern techniques allow to associate bioelectrical signals with specific ion channel proteins in the cell membrane that are central to embryogenesis, regeneration, and tumorigenesis. As a complementary approach to the usual biochemical description, we have studied four biophysical questions: (i) how can single-cell bioelectrical states be established; (ii) how can a change in the cell potential caused by a transient perturbation of the cell state be maintained after the stimulus is gone (bioelectrical memory); (iii) how can a single-cell contribute to the control of multicellular ensembles based on the spatio-temporal pattern of electrical potentials; and (iv) how can oscillatory patterns arise from the single-cell bioelectrical dynamics. Experimentally, endogenous bioelectric gradients have emerged as instructive agents for morphogenetic processes. In this context, the simulations can guide new procedures that may allow a distributed control of the multicellular ensemble.

Electrochemical patterns during Drosophila oogenesis: ion-transport mechanisms generate stage-specific gradients of pH and membrane potential in the follicle-cell epithelium

Background

Alterations of bioelectrical properties of cells and tissues are known to function as wide-ranging signals during development, regeneration and wound-healing in several species. The Drosophila follicle-cell epithelium provides an appropriate model system for studying the potential role of electrochemical signals, like intracellular pH (pH_i) and membrane potential (V_mem), during development. Therefore, we analysed stage-specific gradients of pH_i and V_mem as well as their dependence on specific ion-transport mechanisms.

Results

Using fluorescent indicators, we found distinct alterations of pH_i- and V_mem-patterns during stages 8 to 12 of oogenesis. To determine the roles of relevant ion-transport mechanisms in regulating pH_i and V_mem and in establishing stage-specific antero-posterior and dorso-ventral gradients, we used inhibitors of Na⁺/H⁺-exchangers and Na⁺-channels (amiloride), V-ATPases (bafilomycin), ATP-sensitive K⁺-channels (glibenclamide), voltage-dependent L-type Ca²⁺-channels (verapamil), Cl⁻-channels (9-anthroic acid) and Na⁺/K⁺/2Cl⁻-cotransporters (furosemide). Either pH_i or V_mem or both parameters were affected by each tested inhibitor. While the inhibition of Na⁺/H⁺-exchangers (NHE) and amiloride-sensitive Na⁺-channels or of V-ATPases resulted in relative acidification, inhibiting the other ion-transport mechanisms led to relative alkalisation. The most prominent effects on pH_i were obtained by inhibiting Na⁺/K⁺/2Cl⁻-cotransporters or ATP-sensitive K⁺-channels. V_mem was most efficiently hyperpolarised by inhibiting voltage-dependent L-type Ca²⁺-channels or ATP-sensitive K⁺-channels, whereas the impact of the other ion-transport mechanisms was smaller. In case of very prominent effects of inhibitors on pH_i and/or V_mem, we also found strong influences on the antero-posterior and dorso-ventral pH_i- and/or V_mem-gradients. For example, inhibiting ATP-sensitive K⁺-channels strongly enhanced both pH_i-gradients (increasing alkalisation) and reduced both V_mem-gradients (increasing hyperpolarisation). Similarly, inhibiting Na⁺/K⁺/2Cl⁻-cotransporters strongly enhanced both pH_i-gradients and reduced the antero-posterior V_mem-gradient. To minor extents, both pH_i-gradients were enhanced and both V_mem-gradients were reduced by inhibiting voltage-dependent L-type Ca²⁺-channels, whereas only both pH_i-gradients were reduced (increasing acidification) by inhibiting V-ATPases or NHE and Na⁺-channels.

Conclusions

Our data show that in the Drosophila follicle-cell epithelium stage-specific pH_i- and V_mem-gradients develop which result from the activity of several ion-transport mechanisms. These gradients are supposed to represent important bioelectrical cues during oogenesis, e.g., by serving as electrochemical prepatterns in modifying cell polarity and cytoskeletal organisation.

Electronic supplementary material

The online version of this article (10.1186/s12861-019-0192-x) contains supplementary material, which is available to authorized users.

Connexin 43 maintains tissue polarity and regulates mitotic spindle orientation in the breast epithelium

Cell-cell communication is essential for tissue homeostasis, but its contribution to disease prevention remains to be understood. We demonstrate the involvement of connexin 43 (Cx43, also known as GJA1) and related gap junction in epithelial homeostasis, illustrated by polarity-mediated cell cycle entry and mitotic spindle orientation (MSO). Cx43 localization is restricted to the apicolateral membrane of phenotypically normal breast luminal epithelial cells in 3D culture and in vivo Chemically induced blockade of gap junction intercellular communication (GJIC), as well as the absence of Cx43, disrupt the apicolateral distribution of polarity determinant tight junction marker ZO-1 (also known as TJP1) and lead to random MSO and cell multilayering. Induced expression of Cx43 in cells that normally lack this protein reestablishes polarity and proper MSO in 3D culture. Cx43-directed MSO implicates PI3K-aPKC signaling, and Cx43 co-precipitates with signaling node proteins β-catenin (CTNNB1) and ZO-2 (also known as TJP2) in the polarized epithelium. The distribution of Cx43 is altered by pro-inflammatory breast cancer risk factors such as leptin and high-fat diet, as shown in cell culture and on tissue biopsy sections. The control of polarity-mediated quiescence and MSO may contribute to the tumor-suppressive role of Cx43.

Synchronization of Bioelectric Oscillations in Networks of Nonexcitable Cells: From Single-Cell to Multicellular States

Biological networks use collective oscillations for information processing tasks. In particular, oscillatory membrane potentials have been observed in nonexcitable cells and bacterial communities where specific ion channel proteins contribute to the bioelectric coordination of large populations. We aim at describing theoretically the oscillatory spatiotemporal patterns that emerge at the multicellular level from the single-cell bioelectric dynamics. To this end, we focus on two key questions: (i) What single-cell properties are relevant to multicellular behavior? (ii) What properties defined at the multicellular level can allow an external control of the bioelectric dynamics? In particular, we explore the interplay between transcriptional and translational dynamics and membrane potential dynamics in a model multicellular ensemble, describe the spatiotemporal patterns that arise when the average electric potential allows groups of cells to act as a coordinated multicellular patch, and characterize the resulting synchronization phenomena. The simulations concern bioelectric networks and collective communication across different scales based on oscillatory and synchronization phenomena, thus shedding light on the physiological dynamics of a wide range of endogenous contexts across embryogenesis and regeneration.

Neural control of body-plan axis in regenerating planaria

Control of axial polarity during regeneration is a crucial open question. We developed a quantitative model of regenerating planaria, which elucidates self-assembly mechanisms of morphogen gradients required for robust body-plan control. The computational model has been developed to predict the fraction of heteromorphoses expected in a population of regenerating planaria fragments subjected to different treatments, and for fragments originating from different regions along the anterior-posterior and medio-lateral axis. This allows for a direct comparison between computational and experimental regeneration outcomes. Vector transport of morphogens was identified as a fundamental requirement to account for virtually scale-free self-assembly of the morphogen gradients observed in planarian homeostasis and regeneration. The model correctly describes altered body-plans following many known experimental manipulations, and accurately predicts outcomes of novel cutting scenarios, which we tested. We show that the vector transport field coincides with the alignment of nerve axons distributed throughout the planarian tissue, and demonstrate that the head-tail axis is controlled by the net polarity of neurons in a regenerating fragment. This model provides a comprehensive framework for mechanistically understanding fundamental aspects of body-plan regulation, and sheds new light on the role of the nervous system in directing growth and form.

Planarian regeneration as a model of anatomical homeostasis: Recent progress in biophysical and computational approaches

Planarian behavior, physiology, and pattern control offer profound lessons for regenerative medicine, evolutionary biology, morphogenetic engineering, robotics, and unconventional computation. Despite recent advances in the molecular genetics of stem cell differentiation, this model organism's remarkable anatomical homeostasis provokes us with truly fundamental puzzles about the origin of large-scale shape and its relationship to the genome. In this review article, we first highlight several deep mysteries about planarian regeneration in the context of the current paradigm in this field. We then review recent progress in understanding of the physiological control of an endogenous, bioelectric pattern memory that guides regeneration, and how modulating this memory can permanently alter the flatworm's target morphology. Finally, we focus on computational approaches that complement reductive pathway analysis with synthetic, systems-level understanding of morphological decision-making. We analyze existing models of planarian pattern control and highlight recent successes and remaining knowledge gaps in this interdisciplinary frontier field.

Decoding Calcium Signaling Dynamics during Drosophila Wing Disc Development

The robust specification of organ development depends on coordinated cell-cell communication. This process requires signal integration among multiple pathways, relying on second messengers such as calcium ions. Calcium signaling encodes a significant portion of the cellular state by regulating transcription factors, enzymes, and cytoskeletal proteins. However, the relationships between the inputs specifying cell and organ development, calcium signaling dynamics, and final organ morphology are poorly understood. Here, we have designed a quantitative image-analysis pipeline for decoding organ-level calcium signaling. With this pipeline, we extracted spatiotemporal features of calcium signaling dynamics during the development of the Drosophila larval wing disc, a genetic model for organogenesis. We identified specific classes of wing phenotypes that resulted from calcium signaling pathway perturbations, including defects in gross morphology, vein differentiation, and overall size. We found four qualitative classes of calcium signaling activity. These classes can be ordered based on agonist stimulation strength Gαq-mediated signaling. In vivo calcium signaling dynamics depend on both receptor tyrosine kinase/phospholipase C γ and G protein-coupled receptor/phospholipase C β activities. We found that spatially patterned calcium dynamics correlate with known differential growth rates between anterior and posterior compartments. Integrated calcium signaling activity decreases with increasing tissue size, and it responds to morphogenetic perturbations that impact organ growth. Together, these findings define how calcium signaling dynamics integrate upstream inputs to mediate multiple response outputs in developing epithelial organs.

The Role of Early Bioelectric Signals in the Regeneration of Planarian Anterior/Posterior Polarity

Axial patterning during planarian regeneration relies on a transcriptional circuit that confers distinct positional information on the two ends of an amputated fragment. The earliest known elements of this system begin demarcating differences between anterior and posterior wounds by 6 h postamputation. However, it is still unknown what upstream events break the axial symmetry, allowing a mutual repressor system to establish invariant, distinct biochemical states at the anterior and posterior ends. Here, we show that bioelectric signaling at 3 h is crucial for the formation of proper anterior-posterior polarity in planaria. Briefly manipulating the endogenous bioelectric state by depolarizing the injured tissue during the first 3 h of regeneration alters gene expression by 6 h postamputation and leads to a double-headed phenotype upon regeneration despite confirmed washout of ionophores from tissue. These data reveal a primary functional role for resting membrane potential taking place within the first 3 h after injury and kick-starting the downstream pattern of events that elaborate anatomy over the following 10 days. We propose a simple model of molecular-genetic mechanisms to explain how physiological events taking place immediately after injury regulate the spatial distribution of downstream gene expression and anatomy of regenerating planaria.

Cell-cell bioelectrical interactions and local heterogeneities in genetic networks: a model for the stabilization of single-cell states and multicellular oscillations

Genetic networks operate in the presence of local heterogeneities in single-cell transcription and translation rates. Bioelectrical networks and spatio-temporal maps of cell electric potentials can influence multicellular ensembles. Could cell-cell bioelectrical interactions mediated by intercellular gap junctions contribute to the stabilization of multicellular states against local genetic heterogeneities? We theoretically analyze this question on the basis of two well-established experimental facts: (i) the membrane potential is a reliable read-out of the single-cell electrical state and (ii) when the cells are coupled together, their individual cell potentials can be influenced by ensemble-averaged electrical potentials. We propose a minimal biophysical model for the coupling between genetic and bioelectrical networks that associates the local changes occurring in the transcription and translation rates of an ion channel protein with abnormally low (depolarized) cell potentials. We then analyze the conditions under which the depolarization of a small region (patch) in a multicellular ensemble can be reverted by its bioelectrical coupling with the (normally polarized) neighboring cells. We show also that the coupling between genetic and bioelectric networks of non-excitable cells, modulated by average electric potentials at the multicellular ensemble level, can produce oscillatory phenomena. The simulations show the importance of single-cell potentials characteristic of polarized and depolarized states, the relative sizes of the abnormally polarized patch and the rest of the normally polarized ensemble, and intercellular coupling.

The minimal gap-junction network among melanophores and xanthophores required for stripe-pattern formation in zebrafish

Connexin39.4 (Cx39.4) and Connexin41.8 (Cx41.8), two gap-junction proteins expressed in both melanophores and xanthophores, are critical for the intercellular communication among pigment cells that is necessary for generating the stripe pigment pattern of zebrafish. We previously characterized the gap-junction properties of Cx39.4 and Cx41.8, but how these proteins contribute to stripe formation remains unclear; this is because distinct types of connexins potentially form heteromeric gap junctions, which precludes accurate elucidation of individual connexin functions in vivo. Here, by arranging Cx39.4 and Cx41.8 expression in pigment cells, we identified the simplest gap-junction network required for stripe generation: Cx39.4 expression in melanophores is required but expression in xanthophores is not necessary for stripe patterning, whereas Cx41.8 expression in xanthophores is sufficient for the patterning, and Cx41.8 expression in melanophores might stabilize the stripes. Moreover, patch-clamp recordings revealed that Cx39.4 gap junctions exhibit spermidine-dependent rectification property. Our results suggest that Cx39.4 facilitates the critical cell-cell interactions between melanophores and xanthophores that mediate a unidirectional activation-signal transfer from xanthophores to melanophores, which is essential for melanophore survival.

Intercellular Connectivity and Multicellular Bioelectric Oscillations in Nonexcitable Cells: A Biophysical Model

Bioelectricity is emerging as a crucial mechanism for signal transmission and processing from the single-cell level to multicellular domains. We explore theoretically the oscillatory dynamics that result from the coupling between the genetic and bioelectric descriptions of nonexcitable cells in multicellular ensembles, connecting the genetic prepatterns defined over the ensemble with the resulting spatio-temporal map of cell potentials. These prepatterns assume the existence of a small patch in the ensemble with locally low values of the genetic rate constants that produce a specific ion channel protein whose conductance promotes the cell-polarized state (inward-rectifying channel). In this way, the short-range interactions of the cells within the patch favor the depolarized membrane potential state, whereas the long-range interaction of the patch with the rest of the ensemble promotes the polarized state. The coupling between the local and long-range bioelectric signals allows a binary control of the patch membrane potentials, and alternating cell polarization and depolarization states can be maintained for optimal windows of the number of cells and the intercellular connectivity in the patch. The oscillatory phenomena emerge when the feedback between the single-cell bioelectric and genetic dynamics is coupled at the multicellular level. In this way, the intercellular connectivity acts as a regulatory mechanism for the bioelectrical oscillations. The simulation results are qualitatively discussed in the context of recent experimental studies.

Dynamic intercellular transport modulates the spatial patterning of differentiation during early neural commitment

The initiation of heterogeneity within a population of phenotypically identical progenitors is a critical event for the onset of morphogenesis and differentiation patterning. Gap junction communication within multicellular systems produces complex networks of intercellular connectivity that result in heterogeneous distributions of intracellular signaling molecules. In this study, we investigate emergent systems-level behavior of the intercellular network within embryonic stem cell (ESC) populations and corresponding spatial organization during early neural differentiation. An agent-based model incorporates experimentally-determined parameters to yield complex transport networks for delivery of pro-differentiation cues between neighboring cells, reproducing the morphogenic trajectories during retinoic acid-accelerated mouse ESC differentiation. Furthermore, the model correctly predicts the delayed differentiation and preserved spatial features of the morphogenic trajectory that occurs in response to intercellular perturbation. These findings suggest an integral role of gap junction communication in the temporal coordination of emergent patterning during early differentiation and neural commitment of pluripotent stem cells.

Bioelectrical control of positional information in development and regeneration: A review of conceptual and computational advances

Positional information describes pre-patterns of morphogenetic substances that alter spatio-temporal gene expression to instruct development of growth and form. A wealth of recent data indicate bioelectrical properties, such as the transmembrane potential (Vmem), are involved as instructive signals in the spatiotemporal regulation of morphogenesis. However, the mechanistic relationships between Vmem and molecular positional information are only beginning to be understood. Recent advances in computational modeling are assisting in the development of comprehensive frameworks for mechanistically understanding how endogenous bioelectricity can guide anatomy in a broad range of systems. Vmem represents an extraordinarily strong electric field (∼1.0 × 106 V/m) active over the thin expanse of the plasma membrane, with the capacity to influence a variety of downstream molecular signaling cascades. Moreover, in multicellular networks, intercellular coupling facilitated by gap junction channels may induce directed, electrodiffusive transport of charged molecules between cells of the network to generate new positional information patterning possibilities and characteristics. Given the demonstrated role of Vmem in morphogenesis, here we review current understanding of how Vmem can integrate with molecular regulatory networks to control single cell state, and the unique properties bioelectricity adds to transport phenomena in gap junction-coupled cell networks to facilitate self-assembly of morphogen gradients and other patterns. Understanding how Vmem integrates with biochemical regulatory networks at the level of a single cell, and mechanisms through which Vmem shapes molecular positional information in multicellular networks, are essential for a deep understanding of body plan control in development, regeneration and disease.

Staying in shape: Planarians as a model for understanding regenerative morphology

A key requirement of tissue/organ regeneration is the ability to induce appropriate shape in situ. Regenerated structures need to be integrated with pre-existing ones, through the combined regulation of new tissue growth and the scaling of surrounding tissues. This requires a tightly coordinated control of individual cell functions such as proliferation and stem cell differentiation. While great strides have been made in elucidating cell growth and differentiation mechanisms, how overall shape is generated during regeneration remains unknown. This is because a significant gap remains in our understanding of how cell behaviors are coordinated at the level of tissues and organs. The highly regenerative planarian flatworm has emerged as an important model for defining and understanding regenerative shape mechanisms. This review provides an overview of the main processes known to regulate tissue and animal shape during planarian regeneration: adult stem cell regulation, the reestablishment of body axes, tissue remodeling in pre-existing structures, organ scaling and the maintenance of body proportion, and the bioelectrical regulation of animal morphology. In order for the field to move forward, it will be necessary to identify shape mutants as a means to uncover the molecular mechanisms that synchronize all these separate processes to produce the worm's final regenerative shape. This knowledge will also aid efforts to define the mechanisms that control the termination of regenerative processes.

The Bioelectric Code: Reprogramming Cancer and Aging From the Interface of Mechanical and Chemical Microenvironments

Cancer is a complex, heterogeneous group of diseases that can develop through many routes. Broad treatments such as chemotherapy destroy healthy cells in addition to cancerous ones, but more refined strategies that target specific pathways are usually only effective for a limited number of cancer types. This is largely due to the multitude of physiological variables that differ between cells and their surroundings. It is therefore important to understand how nature coordinates these variables into concerted regulation of growth at the tissue scale. The cellular microenvironment might then be manipulated to drive cells toward a desired outcome at the tissue level. One unexpected parameter, cellular membrane voltage (Vm), has been documented to exert control over cellular behavior both in culture and in vivo. Manipulating this fundamental cellular property influences a remarkable array of organism-wide patterning events, producing striking outcomes in both tumorigenesis as well as regeneration. These studies suggest that Vm is not only a key intrinsic cellular property, but also an integral part of the microenvironment that acts in both space and time to guide cellular behavior. As a result, there is considerable interest in manipulating Vm both to treat cancer as well as to regenerate organs damaged or deteriorated during aging. However, such manipulations have produced conflicting outcomes experimentally, which poses a substantial barrier to understanding the fundamentals of bioelectrical reprogramming. Here, we summarize these inconsistencies and discuss how the mechanical microenvironment may impact bioelectric regulation.